OBJECTIVE: To evaluate the relative contribution of heritable and environmental factors to birthweight of the infant rhesus monkey. RESULTS A retrospective analysis of 25 years of breeding and births was conducted to develop a predictive model of factors that influence birthweight. Across this time period, there was a significant increase in infant birthweight, but the increases were relatively more evident in female than in male offspring. Moreover, there were distinct matrilines that showed this increase more than others. The relationship reflected a high correlation between maternal weight gain during pregnancy and infant birthweight. Specifically, those female matrilines that were prone to show larger weight gain during pregnancy were the ones to bear heavier offspring. We believe this association reflects that the transgenerational passage of a uterine constraint mechanism from mothers to their daughters. FUTURE DIRECTIONS This hypothesis will now be tested in prospective studies, and further statistical modelling of the existing data base. KEY WORDS birthweight, pregnancy, breeding